NEURONAL RESPONSES IN VISUAL AREAS MT AND MST DURING SMOOTH-PURSUIT TARGET SELECTION

We recorded the activity of single neurons in the middle temporal (MT) and middle superior temporal (MST) visual areas in two macaque monkey s while the animals performed a smooth pursuit target selection task. The monkeys were presented with two moving stimuli of different colors and were trained to initiate smooth pursuit to the stimulus that matc hed the color of a previously given cue. We designed these experiments so that we could separate the component of the neuronal response that was driven by the visual stimulus from an extraretinal component that predicted the color or direction of the selected target. We found tha t for all cells in MT and MST the response was primarily determined by the visual stimulus. However, 14% (8 of 58) of MT neurons and 26% (22 of 84) of MST neurons had a small predictive component that was signi ficant at the P less than or equal to 0.05 level. In some cells, the p redictive component was clearly related to the color of the intended t arget, but more often it was correlated with the direction of the targ et. We have previously documented a systematic shift in the latency of smooth pursuit that depends on the relative direction of motion of th e two stimuli. We found that neither the latency nor the amplitude of neuronal responses in MT or MST was correlated with behavioral latency . These results are consistent with a model for target selection in wh ich a weak selection bias for the intended target is amplified by a co mpetitive network that suppresses motion signals related to the nonint ended stimulus. It is possible that the predictive component of neuron al responses in MT and MST contributes to the selection bias. However, the strength of the selection bias in MT and MST is not sufficient to account for the high degree of selectivity shown by pursuit behavior.